1. Field of the Invention
The present invention relates to a bar-shaped vibration driven motor or actuator and, more particularly, to a bar-shaped vibration driven motor suitable for use in optical equipment such as cameras, and OA equipment such as printers.
2. Related Background Art
A bar-shaped vibration driven motor is basically constituted by a bar-shaped vibration member 1, and a rotor 2 contacting the end face of the vibration member 1, as shown in FIG. 2. When a positional phase difference among piezo-electric elements 1b11, 1b12, 1b21, and 1b22 of the vibration member 1, and a temporal phase difference of an applied AC voltage for an ultrasonic wave are properly selected, surface portions of the end face, which serves as the driving surface of the vibration member 1, are caused to follow a circular or elliptic motion, thereby rotating the rotor 2 contacting the driving surface.
In the vibration member, the driving piezo-electric elements 1b11, 1b12, 1b21, and 1b22, and a vibration detection piezo-electric element 1b3 are arranged between columnar vibration member structural bodies 1a1 and 1a2, which are formed of a material such as metals (e.g., Bs, SUS, aluminum, and the like) causing less vibration attenuation. Electrode plates 1c3 to 1c6 are arranged between each pair of adjacent piezo-electric elements. A fastening bolt 3 having a male screw thread is inserted from the side of the vibration member structural body 1a2, and is threadably engaged with a female screw portion of the vibration member structural body 1a2 to clamp and fix the piezo-electric elements therebetween, thus constituting an integrated vibration member.
The rotor 2 is press contacted to the driving surface of the vibration member 1 via a spring case 5a by the biasing force of a compression spring 5 so as to obtain a frictional force. A rotary output member 6 frictionally contacts the spring case 5a. The member 6 has a gear portion on its outer circumferential surface, and is meshed with a gear (not shown) to transmit the rotational force of the rotor 2 to an external mechanism. The rotary output member 6 has a bearing 7.
Therefore, when the driving surface of the vibration member 1 makes a circular or elliptic motion, since the rotor 2 contacts near the peaks of the locus of, e.g., the elliptic motion, it is frictionally driven at a speed substantially proportional to the tangential speed. In order to increase the rotational speed of the motor, the vibration amplitude on the driving surface must be increased.
Most of the energy losses in the vibration member are internal frictional losses caused by strain in the vibration member caused by the vibration, and depend on the total sum of the strains.
For this reason, in order to increase the rotational speed of the motor, and to reduce the energy losses, it is desirable to increase the vibration amplitude of only a portion of the vibration member near the contact portion.
Thus, the present applicant has proposed a vibration 10 member in which a circumferential groove 1d is formed in the vibration member 1 so as to increase the vibration amplitude of only a portion of the vibration member near the contact portion.
FIGS. 3A and 3B show radial displacement distributions of the shaft portion of the vibration member depending on the presence/absence of a circumferential groove 1d of the vibration member 1. FIG. 3A shows the case of a vibration member having no circumferential groove 1d, and FIG. 3B shows the case of a vibration member having a circumferential groove 1d. As can be seen from FIG. 3B, in the vibration member having a circumferential groove 1d, the rigidity of the portion of the vibration member at the circumferential groove 1d is lowered, and a large displacement is obtained at the side of the contact portion with the rotor as the driving surface.
From this fact, if the vibration members of FIGS. 3A and 3B are designed to have the same displacement at the contact portion with the rotor, then the displacements in other portions of the vibration member shown in FIG. 3B are generally smaller than the corresponding portions of the vibration member shown in FIG. 3A. As a result, the total sum of strains, i.e., the internal loss in the vibration member can be reduced.
A bar-shaped vibration driven motor utilizes orthogonal bending natural vibrations in two directions (x- and y-directions) as the driving force.
Therefore, it is impossible to obtain large amplitudes in both directions unless the two natural frequencies are substantially equal to each other. In this case, the locus of the surface portions of the vibration member is considerably shifted from a circular motion, and undesirably becomes closer to a linear motion.
As a result, a high rotational speed cannot be obtained as a motor output, resulting in poor efficiency.
Note that the two natural frequencies can be originally matched with each other by design calculations.
However, in practice, the two natural frequencies have a difference (to be referred to as .DELTA.f hereinafter) therebetween, and the difference varies depending on individual vibration members.
As a result, the motor performance varies depending on individual motors.
Upon examination of the cause for the variation, it has been found that when a screw portion 4 for clamping and fixing the upper and lower vibration member structural bodies 1a1 and 1a2 (see FIG. 2) is present near the circumferential groove 1d, the variation .DELTA.f becomes large.
It is believed that the above-mentioned fact is caused by the presence of strong and weak meshing portions due to machining errors of the male and female screw portions. More specifically, this causes a nonuniform rigidity, and the natural frequencies have a difference therebetween depending on a rigidity difference in the x- and y-directions of the vibration member.
As can be seen from FIG. 3B, at the position of the circumferential groove 1d having a low rigidity, a change (.theta..sub.2 -.theta..sub.1) in inclination angle of a vibration mode is large, and a large strain occurs.
Therefore, the rigidity difference at this position tends to appear as a difference between the bending natural frequencies.